1. Technical Field
The present invention relates to a method and an apparatus for laser marking, and a method and apparatus for detecting a mark, in particular, to a method and an apparatus of laser marking a subject, such as a semiconductor wafer, and a method and apparatus for detecting a mark formed on a subject, such as a semiconductor wafer.
2. Description of the Related Art
Japanese Patent Disclosure (Kokai) No. 11-33752 shows a laser marking method to make a marking of good visibility on an optical material, such as a glass for a liquid crystal display, without cracking the optical material.
A laser marker shown in this disclosure has a laser apparatus emitting a pulse laser beam having 30 through 100 μm width. A polygon mirror swings the pulse laser beam so as to ablate a surface of the optical material. Thus, a concave portion visually recognized as a marking is formed on the optical material.
Japanese Patent Disclosure (Kokai) No. 11-260675 shows another conventional laser marking method to make a dot marking on a semiconductor wafer. The method prevents the adhesion of melted splashing particles on a semiconductor wafer caused during laser marking and ensures visibility by keeping stable all the time the shapes of the dot marks, even after being subjected to a number of treatments during a number of steps in a semiconductor device manufacturing process.
In this method, a transparent thin film is formed at least in a dot mark forming region of a surface of a semiconductor wafer. The dot mark forming region is irradiated with a laser beam having a wavelength through the transparent thin film. The laser beam transmitted through the transparent thin film melts and deforms the dot mark forming region to form dot marks. At the same time, the heat which is generated when the mark forming region is melted is used to change the transparent thin film into the same shape as the dot marks without causing the destruction of the transparent thin film. The thin film protects the dot marks from the adhesion of melted splashing particles and ensures the visibility of the dot marks.
FIG. 5 shows another conventional marking apparatus to make a marking on a subject such as a semiconductor wafer using a laser beam having a Gaussian intensity distribution. A laser light source 1 emits a laser beam toward a galvano-mirror 2 which serves as an incident pupil. Galvano-mirror 2 includes a moveable coil (not shown) placed in a magnetic field. Flowing current generates an electromagnetic force according to an amount of the current, and rotates the moveable coil with a small reflective mirror attached to the axis thereof. The angle of the reflective mirror is adjustable by controlling the amount of the current. A laser beam reflected off galvano-mirror 2, is focused on a back surface of semiconductor wafer 4 using focus lens 3 (f-θ lens) so that dot marks are made. Depths of the dot marks are very deep, i.e., around 5 μm, so these dot marks are called hard marks. Even though an intensity of the laser beam is trying to be kept as minimum as possible so as not to damage the dot marks and make the dot marks connect to each other, semiconductor wafer 4 sometimes cracks or weakens owing to the laser beam. Moreover, making hard marks needs such a high intensity of a laser beam that semiconductor wafer 4 cannot help ablating, which contaminates an area around the hard marks, and/or deteriorates the flatness of the semiconductor wafer, which has a negative influence on the process of forming a transistor, in particular on lithography processes, on semiconductor wafer 4.
In order to avoid these problems, a soft marking method which uses a laser beam having a lower intensity is also known. However, the soft marking method forms a fusion mark of submicron scale, which deteriorates visibility of a soft mark.
It is desirable to provide a laser marking apparatus and method which makes a shallow marking with a good visibility because fine lithography in a semiconductor manufacturing process needs a flat wafer.